Direction selectivity is a complex receptive field property displayed by many neurons in the visual system. Each cell fires a series of action potentials to motion of a bar stimulus moved across the receptive field in a "preferred" direction. while movement of the bar in the opposite direction, termed the "null" direction evokes little or no response. The rabbit retina provides a rich source of direction selective ganglion cells, yet little is known about the synaptic input to these cells. Furthermore, the directional preferences of these cells are not obvious from their dendritic morphologies. I have found that certain types of excitatory amino acid (EAA) receptors appear to be critical to the function of the direction preferring mechanism of direction selective ganglion cells. In the presence of two excitatory amino acid receptor antagonists, NBQX and DNQX, the directional preference of these ganglion cells is lost to bar stimuli, and the ganglion cells respond equally to bar movement in both directions. Many theories on direction selectivity propose that a bar stimulus moving in the "null" direction of the receptive field activates a spatially asymmetric, feedforward. sustained inhibition. This preceding bar stimulus inhibition "shunts out" any bar excitation that follows it. Intracellular recordings in the rabbit retina reveal that the light evoked responses of many inner retinal neurons lose their sustained portions of the light evoked response in the presence of NBQX, and respond transiently to light. The hypothesis that the loss of direction selectivity in the "null" direction observed in NBQX is due to loss of a sustained feedforward inhibition will be tested by intracellular recording from neurons involved in direction selective pathways. Particular attention will be focused on the effects of NBQX and DNQX on the excitatory postsynaptic potentials of direction selective ganglion cells, and the starburst (cholinergic) amacrine cells thought to be presynaptic to these cells. The pharmacological nature of the excitatory amino acid synaptic input to direction selective ganglion cells will be characterized in retinal slices. These studies will provide new insights into the role of amacrine and bipolar cell function in the direction selective mechanism of these ganglion cells, and provide a more quantitative framework for improved models of direction selectivity in the visual system.